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Texture and shape memory property of annealed Ti50Ni25Cu25 ribbons
Materials Science and Engineering A 425 (2006) 268–271
Texture and shape memory property of annealed Ti50Ni25Cu25 ribbons G.P. Cheng a , Z.L. Xie a,∗ , Y. Liu b a
b
School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singapore Received 3 March 2006; received in revised form 23 March 2006; accepted 24 March 2006
Abstract Texture development and shape recovery strain of the annealed Ti50 Ni25 Cu25 ribbons were studied by using XRD and TMA. A pure strong [2 1 1] fiber texture existed in the 450 ◦ C-annealed sample. Annealing at 550 ◦ C and above changed the texture component in B19 martensite to (1 1 1)[7 1 5] and caused the formation of B11-TiCu precipitates. Shape recovery strain of the annealed ribbons decreased with the increase of annealing temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Shape memory alloy; Ti–Ni–Cu ribbon; Texture; Shape recovery strain
1. Introduction Shape memory effect (SME) is a unique property of shape memory alloys (SMAs). TiNi alloys are important shape memory alloys with good SME. It is reported that shape recoverable strain induced by martensitic transformation showed strong orientation dependence in Ti–Ni alloys [1,2]. Melt-spinning technique is widely used to produce Ti–Ni–Cu alloys with Cu content more than 20%. The rapid quenching during spinning causes grain refinement and changes grain morphology from equi-axial to columnar [3]. This results in the formation of texture and affects the mechanical properties. The shape memory properties of Ti50 Ni25 Cu25 melt-spun ribbons were studied by Santamarta et al. [4,5] and Liu [6]. Santamarta et al. [4,5] compared shape recovery strain of Ti–Ni alloys with that of Ti–Ni–Cu ribbons and found textures affected SME. Liu [6] found it had good super-elasticity at lower annealing temperature, but poor mechanical properties at higher annealing temperature. Little work has been devoted to the effect of annealing on texture development in the ribbon. In this paper, the effect of annealing temperature on the evolution of texture and shape memory properties of Ti50 Ni25 Cu25 ribbons was studied ∗
Corresponding author. Tel.: +65 6790 6921; fax: +65 6790 9081. E-mail address: [email protected] (Z.L. Xie).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.054
by using XRD and TMA under different tensile loads. The relationship between texture components and shape memory effect was investigated. 2. Experimental Ti50 Ni25 Cu25 (at.%) ribbon was fabricated by melt-spinning method. It had an amorphous initial structure. Crystallization temperature was measured to be 450 ◦ C [7]. The as-spun ribbon was annealed at different temperatures from 450 to 750 ◦ C with an interval of 50 ◦ C for 15 min, followed by water quenching to room temperature (RT). Incomplete pole figures were measured at RT using Cu K␣ radiation for the reflection method with tilting angle from 30◦ to 90◦ . Sample size was 18 mm × 18 mm. In pole figure measurement, RD is the spinning direction and TD is perpendicular to the spinning direction. Texture components were determined by comparing the measured pole figure with the standard stereographic projection of the corresponding crystal structure. Shape recovery strain of the annealed ribbons was determined by measuring the change of elongation versus temperature under constant tensile load, using thermo-mechanical analyzer (TMA). Load was applied along the spinning direction. Tensile test sample was 1 mm in width and 15 mm in length.
G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271
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Fig. 2. {1 1 1} pole figure of Ti50 Ni25 Cu25 ribbon annealed at 500 ◦ C.
Fig. 1. Pole figures of the 450 ◦ C-annealed Ti50 Ni25 Cu25 ribbon.
3. Results 3.1. Texture in the 450 ◦ C-annealed ribbon After annealing at 450 ◦ C for 15 min, all the Ti50 Ni25 Cu25 ribbons showed a crystalline structure. It was reported in our previous study [7] that only B19 martensite existed in the 450 ◦ Cannealed ribbon. The high intensity ratio of 1 1 1/0 0 2, 1 1 1/0 2 2 and 1 1 1/1 3 1 revealed a strong preferred orientation developed in this crystallized ribbon. The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {0 2 2} of B19 are shown in Fig. 1. The center of the pole figures corresponds to the normal direction of the sample surface (ND). The pole figures showed how the respective pole density distributed in the specimen coordinate system RD–TD–ND. The maximum relative pole intensity Imax was indicated in the figures. From Fig. 1, it is known that the pole figures of {0 0 2}, {0 1 2} and {0 2 2} planes showed a considerably uniform pole density distribution. Only {1 1 1} pole figure showed a clustered density distribution with two peaks symmetrically located within 5◦ from ND. The maximum relative intensity was 31.7. By comparing the measured pole figures with the standard stereographic projection of B19 lattice, a [2 1 1] fiber texture was determined. The ribbon surface was nearly parallel to {1 1 1} planes of B19 (about 5◦ away). This indicates that B19 martensite grains grew column-like in the ribbon. And they distributed rotationally around [2 1 1] direction, which is ND of the ribbon plane.
above 500 ◦ C showed a mixture of B19 martensite and B11-TiCu phase. The position of diffraction peaks did not change with annealing temperature, which means lattice parameters of B19 and B11-TiCu phase were identical for all the annealed ribbons. However, the relative intensity of diffraction peaks changed with the increase of annealing temperature. At lower annealing temperature ribbons consisted of mainly B19 phase with (1 1 1) preferential orientation. High temperature annealing weakened the texture in B19 phase and assisted the formation of B11-TiCu phase. Texture development in the annealed ribbon was investigated by measuring pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1} planes of B19 martensite. In the ribbon annealed at 500 ◦ C, only {1 1 1} pole figure showed a clustered distribution around ND (shown in Fig. 2). The pole figures of {0 0 2}, {0 1 2} and {1 3 1} showed random distribution. Compared with the pole figures of the 450 ◦ Cannealed ribbon, the position of the maximum pole density of {1 1 1} in Fig. 2 did not change. But the maximum relative intensity of {1 1 1} pole decreased from 31.7 to 9.5. This indicates that the texture component of the ribbon annealed at 500 ◦ C was still a [2 1 1] fiber texture. But it was weaker than that of the 450 ◦ C-annealed ribbon. Fig. 3 is the pole figures of {1 1 1} and {0 1 2} in the ribbon annealed at 550 ◦ C. It can be seen that the clustered distribution of {1 1 1} is similar to that of the 500 ◦ C-annealed sample. In addition, the pole figure of {0 1 2} showed obvious clustered density distribution with one pole density peak located at 50◦ from ND along RD. The maximum relative intensity of {0 1 2} pole is 36.9. Qualitative analysis revealed that the texture compo-
3.2. Textures development in B19 phase as a function of annealing temperature The evolution of crystal structure with annealing temperature was reported in our previous study [7]. Samples annealed
Fig. 3. Pole figures of Ti50 Ni25 Cu25 ribbon annealed at 550 ◦ C.
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nent in this ribbon was a (1 1 1)[7¯ 1 5] sheet texture. (111) plane of B19 was still nearly parallel to ribbon surface. Therefore, increasing annealing temperature did not change the orientation of (1 1 1) plane of B19 phase, but changed the texture component from fiber to sheet texture. The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1} in the ribbon annealed at 600, 650 and 700 ◦ C were also measured. They shared similar distribution of maximum pole density to those of the 550 ◦ C-annealed ribbon. The maximum relative intensity of {1 1 1} pole in the 600, 650 and 700 ◦ C-annealed ribbons was 5.3, 4.9 and 4.8, respectively. Therefore, the texture component did not change with further annealing: (1 1 1)[7¯ 1 5] sheet texture, but its intensity decreased. It can be seen that texture components and intensity changed with the annealing temperature. Annealing below 550 ◦ C favored [2 1 1] fiber texture, while higher temperature annealing benefited (1 1 1)[7¯ 1 5] sheet texture. But {1 1 1} plane of B19 phase is nearly parallel to the ribbon surface in all the annealed ribbons. Kim et al. [8] reported that the texture development in the annealed cold-rolled Ti–Nb–Ta alloy was due to recrystallization. From the XRD results [7], it is known that B11 TiCu precipitates formed above 550 ◦ C. The change of texture with annealing temperature may be due to the restrained orientation of martensite by the formation of these precipitates. 3.3. Shape recovery strain Thermal cycling tests on the annealed Ti50 Ni25 Cu25 ribbons were carried out under different tensile biasing loads: 6.3, 11.3, 25, 37.5 and 45 MPa. Strain–temperature curves were measured under various constant stresses during a thermal cycle from below Mf (−40 ◦ C) to above Af (120 ◦ C). Fig. 4 shows the strain–temperature curves of the 500 ◦ C-annealed ribbon under different loads. The elongation on cooling and shrinkage on heating indicate that the annealed ribbon exhibited a well-defined shape memory effect. Under a constant stress, the
Fig. 4. TMA curves of Ti50 Ni25 Cu25 ribbon annealed at 500 ◦ C under different loads.
Fig. 5. Shape recovery strain of Ti50 Ni25 Cu25 ribbon annealed at different temperatures under different loads.
specimen started to elongate due to the martensitic transformation at Ms and finished the deformation at Mf upon cooling. While it started to recover the deformation and shrink due to the reverse transformation at As and finished the recovery at Af upon heating. The shape recovery strain was calculated as a difference between the elongation in martensitic and austenite phase. The annealed ribbon showed a very small recovery strain under a lower applied load (0.2% under 6.3 MPa). With the increase of applied load, the shape recovery strain increased. Under 45 MPa, a recoverable strain of 1.5% was obtained. Fig. 5 provides the shape recovery strain as a function of annealing temperatures under different biasing loads. Under a lower load (6.3 MPa), recovery strain decreased slowly with the annealing temperature. It is due to the small volume of reoriented martensite under low stress. When stressed under a higher load (45 MPa), there was a sharp decrease of recovery strain with increasing annealing temperature from 500 to 550 ◦ C. For the ribbon annealed at 450 ◦ C, the recoverable strain was 1.8%. It decreased drastically to 0.38% for the sample annealed at 750 ◦ C. As observed in the texture analysis, the 450 ◦ C-annealed ribbon had a pure sharp [2 1 1] fiber texture with strong intensity. At annealing temperature 550 ◦ C and above, a sheet texture component (1 1 1) [2¯ 1 1] existed and the intensity of texture decreased. In addition, B11-TiCu precipitates formed in the ribbons annealed above 550 ◦ C act as an obstacle to the reorientation of martensite. The combination of texture and precipitate determined the shape recovery strain of the annealed ribbons. Our observations indicate B19 martensite with strong texture and no precipitates was beneficial to shape memory effect. The [2 1 1] fiber texture has its fiber axis perpendicular to the ribbon surface. Crystalline orientation distribution is uniform on the ribbon surface so that the transformation anisotropy is weak. The isotropic transformation strain on the ribbon surface is convenient for designing micro-actuators.
G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271
4. Conclusions
References
Texture evolution in the annealed Ti50 Ni25 Cu25 ribbons was observed. Qualitative analysis of the pole figures revealed that texture components changed with the annealing temperature. Annealed at lower temperature (450 ◦ C), a pure strong [2 1 1] fiber texture existed in the sample. Annealing at 550 ◦ C and above changed texture components to a (1 1 1)[7¯ 1 5] sheet texture. In this study, strong textured B19 martensite without precipitates is beneficial to the shape memory property.
[1] [2] [3] [4] [5] [6] [7] [8]
271
S. Miyazaki, K. Otsuka, C.M. Wayman, Acta Metall. 37 (1989) 1873. H. Inoue, N. Miwa, N. Inakazu, Acta Mater. 44 (1996) 4825. T. Goryczka, H. Morawiec, Mater. Sci. Eng. A 378 (2004) 248. R. Santamarta, E. Cesari, J. Pons, T. Goryczka, Metall. Mater. Trans. A 35A (2004) 761. R. Santamarta, J. Pons, E. Cesari, J. Phys. IV 11 (2001) 351. Y. Liu, Mater. Sci. Eng. A 354 (2003) 286. G.P. Cheng, Z.L. Xie, Y. Liu, J. Alloys Compd., in press. H.Y. Kim, T. Sasaki, K. Okutsu, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 423.
Texture and shape memory property of annealed Ti50Ni25Cu25 ribbons G.P. Cheng a , Z.L. Xie a,∗ , Y. Liu b a
b
School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singapore Received 3 March 2006; received in revised form 23 March 2006; accepted 24 March 2006
Abstract Texture development and shape recovery strain of the annealed Ti50 Ni25 Cu25 ribbons were studied by using XRD and TMA. A pure strong [2 1 1] fiber texture existed in the 450 ◦ C-annealed sample. Annealing at 550 ◦ C and above changed the texture component in B19 martensite to (1 1 1)[7 1 5] and caused the formation of B11-TiCu precipitates. Shape recovery strain of the annealed ribbons decreased with the increase of annealing temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Shape memory alloy; Ti–Ni–Cu ribbon; Texture; Shape recovery strain
1. Introduction Shape memory effect (SME) is a unique property of shape memory alloys (SMAs). TiNi alloys are important shape memory alloys with good SME. It is reported that shape recoverable strain induced by martensitic transformation showed strong orientation dependence in Ti–Ni alloys [1,2]. Melt-spinning technique is widely used to produce Ti–Ni–Cu alloys with Cu content more than 20%. The rapid quenching during spinning causes grain refinement and changes grain morphology from equi-axial to columnar [3]. This results in the formation of texture and affects the mechanical properties. The shape memory properties of Ti50 Ni25 Cu25 melt-spun ribbons were studied by Santamarta et al. [4,5] and Liu [6]. Santamarta et al. [4,5] compared shape recovery strain of Ti–Ni alloys with that of Ti–Ni–Cu ribbons and found textures affected SME. Liu [6] found it had good super-elasticity at lower annealing temperature, but poor mechanical properties at higher annealing temperature. Little work has been devoted to the effect of annealing on texture development in the ribbon. In this paper, the effect of annealing temperature on the evolution of texture and shape memory properties of Ti50 Ni25 Cu25 ribbons was studied ∗
Corresponding author. Tel.: +65 6790 6921; fax: +65 6790 9081. E-mail address: [email protected] (Z.L. Xie).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.054
by using XRD and TMA under different tensile loads. The relationship between texture components and shape memory effect was investigated. 2. Experimental Ti50 Ni25 Cu25 (at.%) ribbon was fabricated by melt-spinning method. It had an amorphous initial structure. Crystallization temperature was measured to be 450 ◦ C [7]. The as-spun ribbon was annealed at different temperatures from 450 to 750 ◦ C with an interval of 50 ◦ C for 15 min, followed by water quenching to room temperature (RT). Incomplete pole figures were measured at RT using Cu K␣ radiation for the reflection method with tilting angle from 30◦ to 90◦ . Sample size was 18 mm × 18 mm. In pole figure measurement, RD is the spinning direction and TD is perpendicular to the spinning direction. Texture components were determined by comparing the measured pole figure with the standard stereographic projection of the corresponding crystal structure. Shape recovery strain of the annealed ribbons was determined by measuring the change of elongation versus temperature under constant tensile load, using thermo-mechanical analyzer (TMA). Load was applied along the spinning direction. Tensile test sample was 1 mm in width and 15 mm in length.
G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271
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Fig. 2. {1 1 1} pole figure of Ti50 Ni25 Cu25 ribbon annealed at 500 ◦ C.
Fig. 1. Pole figures of the 450 ◦ C-annealed Ti50 Ni25 Cu25 ribbon.
3. Results 3.1. Texture in the 450 ◦ C-annealed ribbon After annealing at 450 ◦ C for 15 min, all the Ti50 Ni25 Cu25 ribbons showed a crystalline structure. It was reported in our previous study [7] that only B19 martensite existed in the 450 ◦ Cannealed ribbon. The high intensity ratio of 1 1 1/0 0 2, 1 1 1/0 2 2 and 1 1 1/1 3 1 revealed a strong preferred orientation developed in this crystallized ribbon. The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {0 2 2} of B19 are shown in Fig. 1. The center of the pole figures corresponds to the normal direction of the sample surface (ND). The pole figures showed how the respective pole density distributed in the specimen coordinate system RD–TD–ND. The maximum relative pole intensity Imax was indicated in the figures. From Fig. 1, it is known that the pole figures of {0 0 2}, {0 1 2} and {0 2 2} planes showed a considerably uniform pole density distribution. Only {1 1 1} pole figure showed a clustered density distribution with two peaks symmetrically located within 5◦ from ND. The maximum relative intensity was 31.7. By comparing the measured pole figures with the standard stereographic projection of B19 lattice, a [2 1 1] fiber texture was determined. The ribbon surface was nearly parallel to {1 1 1} planes of B19 (about 5◦ away). This indicates that B19 martensite grains grew column-like in the ribbon. And they distributed rotationally around [2 1 1] direction, which is ND of the ribbon plane.
above 500 ◦ C showed a mixture of B19 martensite and B11-TiCu phase. The position of diffraction peaks did not change with annealing temperature, which means lattice parameters of B19 and B11-TiCu phase were identical for all the annealed ribbons. However, the relative intensity of diffraction peaks changed with the increase of annealing temperature. At lower annealing temperature ribbons consisted of mainly B19 phase with (1 1 1) preferential orientation. High temperature annealing weakened the texture in B19 phase and assisted the formation of B11-TiCu phase. Texture development in the annealed ribbon was investigated by measuring pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1} planes of B19 martensite. In the ribbon annealed at 500 ◦ C, only {1 1 1} pole figure showed a clustered distribution around ND (shown in Fig. 2). The pole figures of {0 0 2}, {0 1 2} and {1 3 1} showed random distribution. Compared with the pole figures of the 450 ◦ Cannealed ribbon, the position of the maximum pole density of {1 1 1} in Fig. 2 did not change. But the maximum relative intensity of {1 1 1} pole decreased from 31.7 to 9.5. This indicates that the texture component of the ribbon annealed at 500 ◦ C was still a [2 1 1] fiber texture. But it was weaker than that of the 450 ◦ C-annealed ribbon. Fig. 3 is the pole figures of {1 1 1} and {0 1 2} in the ribbon annealed at 550 ◦ C. It can be seen that the clustered distribution of {1 1 1} is similar to that of the 500 ◦ C-annealed sample. In addition, the pole figure of {0 1 2} showed obvious clustered density distribution with one pole density peak located at 50◦ from ND along RD. The maximum relative intensity of {0 1 2} pole is 36.9. Qualitative analysis revealed that the texture compo-
3.2. Textures development in B19 phase as a function of annealing temperature The evolution of crystal structure with annealing temperature was reported in our previous study [7]. Samples annealed
Fig. 3. Pole figures of Ti50 Ni25 Cu25 ribbon annealed at 550 ◦ C.
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G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271
nent in this ribbon was a (1 1 1)[7¯ 1 5] sheet texture. (111) plane of B19 was still nearly parallel to ribbon surface. Therefore, increasing annealing temperature did not change the orientation of (1 1 1) plane of B19 phase, but changed the texture component from fiber to sheet texture. The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1} in the ribbon annealed at 600, 650 and 700 ◦ C were also measured. They shared similar distribution of maximum pole density to those of the 550 ◦ C-annealed ribbon. The maximum relative intensity of {1 1 1} pole in the 600, 650 and 700 ◦ C-annealed ribbons was 5.3, 4.9 and 4.8, respectively. Therefore, the texture component did not change with further annealing: (1 1 1)[7¯ 1 5] sheet texture, but its intensity decreased. It can be seen that texture components and intensity changed with the annealing temperature. Annealing below 550 ◦ C favored [2 1 1] fiber texture, while higher temperature annealing benefited (1 1 1)[7¯ 1 5] sheet texture. But {1 1 1} plane of B19 phase is nearly parallel to the ribbon surface in all the annealed ribbons. Kim et al. [8] reported that the texture development in the annealed cold-rolled Ti–Nb–Ta alloy was due to recrystallization. From the XRD results [7], it is known that B11 TiCu precipitates formed above 550 ◦ C. The change of texture with annealing temperature may be due to the restrained orientation of martensite by the formation of these precipitates. 3.3. Shape recovery strain Thermal cycling tests on the annealed Ti50 Ni25 Cu25 ribbons were carried out under different tensile biasing loads: 6.3, 11.3, 25, 37.5 and 45 MPa. Strain–temperature curves were measured under various constant stresses during a thermal cycle from below Mf (−40 ◦ C) to above Af (120 ◦ C). Fig. 4 shows the strain–temperature curves of the 500 ◦ C-annealed ribbon under different loads. The elongation on cooling and shrinkage on heating indicate that the annealed ribbon exhibited a well-defined shape memory effect. Under a constant stress, the
Fig. 4. TMA curves of Ti50 Ni25 Cu25 ribbon annealed at 500 ◦ C under different loads.
Fig. 5. Shape recovery strain of Ti50 Ni25 Cu25 ribbon annealed at different temperatures under different loads.
specimen started to elongate due to the martensitic transformation at Ms and finished the deformation at Mf upon cooling. While it started to recover the deformation and shrink due to the reverse transformation at As and finished the recovery at Af upon heating. The shape recovery strain was calculated as a difference between the elongation in martensitic and austenite phase. The annealed ribbon showed a very small recovery strain under a lower applied load (0.2% under 6.3 MPa). With the increase of applied load, the shape recovery strain increased. Under 45 MPa, a recoverable strain of 1.5% was obtained. Fig. 5 provides the shape recovery strain as a function of annealing temperatures under different biasing loads. Under a lower load (6.3 MPa), recovery strain decreased slowly with the annealing temperature. It is due to the small volume of reoriented martensite under low stress. When stressed under a higher load (45 MPa), there was a sharp decrease of recovery strain with increasing annealing temperature from 500 to 550 ◦ C. For the ribbon annealed at 450 ◦ C, the recoverable strain was 1.8%. It decreased drastically to 0.38% for the sample annealed at 750 ◦ C. As observed in the texture analysis, the 450 ◦ C-annealed ribbon had a pure sharp [2 1 1] fiber texture with strong intensity. At annealing temperature 550 ◦ C and above, a sheet texture component (1 1 1) [2¯ 1 1] existed and the intensity of texture decreased. In addition, B11-TiCu precipitates formed in the ribbons annealed above 550 ◦ C act as an obstacle to the reorientation of martensite. The combination of texture and precipitate determined the shape recovery strain of the annealed ribbons. Our observations indicate B19 martensite with strong texture and no precipitates was beneficial to shape memory effect. The [2 1 1] fiber texture has its fiber axis perpendicular to the ribbon surface. Crystalline orientation distribution is uniform on the ribbon surface so that the transformation anisotropy is weak. The isotropic transformation strain on the ribbon surface is convenient for designing micro-actuators.
G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271
4. Conclusions
References
Texture evolution in the annealed Ti50 Ni25 Cu25 ribbons was observed. Qualitative analysis of the pole figures revealed that texture components changed with the annealing temperature. Annealed at lower temperature (450 ◦ C), a pure strong [2 1 1] fiber texture existed in the sample. Annealing at 550 ◦ C and above changed texture components to a (1 1 1)[7¯ 1 5] sheet texture. In this study, strong textured B19 martensite without precipitates is beneficial to the shape memory property.
[1] [2] [3] [4] [5] [6] [7] [8]
271
S. Miyazaki, K. Otsuka, C.M. Wayman, Acta Metall. 37 (1989) 1873. H. Inoue, N. Miwa, N. Inakazu, Acta Mater. 44 (1996) 4825. T. Goryczka, H. Morawiec, Mater. Sci. Eng. A 378 (2004) 248. R. Santamarta, E. Cesari, J. Pons, T. Goryczka, Metall. Mater. Trans. A 35A (2004) 761. R. Santamarta, J. Pons, E. Cesari, J. Phys. IV 11 (2001) 351. Y. Liu, Mater. Sci. Eng. A 354 (2003) 286. G.P. Cheng, Z.L. Xie, Y. Liu, J. Alloys Compd., in press. H.Y. Kim, T. Sasaki, K. Okutsu, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 423.